The invention relates to a working machine, a method and a control unit for controlling a working machine. The invention also relates to a control element for a working machine.
The invention can be applied on different types of working machine, for example wheel loaders and articulated haulers. Although the invention will be described hereinafter with respect to a wheel loader, the application of the invention is not restricted to this particular application, but the invention may also be used in other vehicles.
A working machine is usually provided with a bucket, container or other type of implement for digging, lifting, carrying and/or transporting a load.
For example, a wheel loader has a lift arm unit for raising and lowering an implement, such as a bucket. The lift arm unit comprises hydraulic cylinders for movement of a load arm and the implement attached to the load arm. Usually a pair of hydraulic cylinders is arranged for raising the load arm and a further hydraulic cylinder is arranged for tilting the implement relative to the load arm.
In addition, the working machine is often articulated frame-steered and has a pair of hydraulic cylinders for turning/steering the working machine by pivoting a front section and a rear section of the working machine relative to each other.
The hydraulic system generally further comprises at least one hydraulic pump, which is arranged to supply hydraulic power, i.e. hydraulic flow and/or hydraulic pressure to the hydraulic cylinders. The hydraulic pump is conventionally driven by an internal combustion engine and provides the hydraulic fluid by means of a control valve in a load sensing (LS) system. The pump is connected to a power take-off which can be located between the internal combustion engine and a transmission arrangement, such as a gear box. The transmission arrangement is in turn connected to e.g. wheels of the working machine for the propulsion thereof. In other words; both the hydraulic pump and the driveline are directly driven by the engine.
The way of operating the hydraulic actuators in a wheel loader is usually performed more or less by “on-off” control. In a loading cycle during lifting and tilting of the bucket, the operator usually uses the maximal available lever stroke. The reason is that the maximal flow provided from the pump (or pumps) is restricted due to the (low) engine speed required by the driveline. Thus, the operator uses all available power to the lift and tilt by actuating the lever for controlling the control valve. The control unit opens the control valve and increases the displacement of the variable pump to achieve the requisite flow and pressure, but the power is still limited since the engine speed is determined by the driveline.
On the other hand, in a working machine with a system where the engine drives a generator that in turn drives an electric motor, preferably via any kind of energy storage, for the propulsion of the working machine, the driveline is not directly connected to the engine. In this system there is actually no such engine speed restriction since the speed of the engine can be high without being in conflict with the speed required by the driveline.
Due to the fact that an operator of a working machine tends to use the same approach as for conventional machines also when driving this kind of machine not having any engine speed restriction caused by the driveline, the fuel consumption would increase as a consequence of using a higher pump speed if the operator was given the ability to increase the pump speed unrestrictedly.
It is desirable to provide a working machine by which the energy losses and thereby the fuel consumption can be reduced.
The invention according to an aspect thereof, is based on the insight that by the provision of a working machine that comprises an internal combustion engine, a generator, an energy storage and an electric motor for driving one or more wheels of the working machine, where the internal combustion engine is arranged to drive the generator and the generator is arranged to supply power to the electric motor via the energy storage, and the working machine further comprises an accelerator for controlling the electric motor, and a hydraulic pump for driving a hydraulic actuator, and particularly where the hydraulic pump has a drive source different from the electric motor and the accelerator is arranged for controlling the speed of the hydraulic pump, the energy losses can be reduced due to the fact that the speed of the drive source of the hydraulic pump can be restricted in a way that is similar to the characteristics of a conventional working machine where both the hydraulic pump and the driveline are directly driven by an internal combustion engine.
The balance between the machine performance and fuel save can be controlled by controlling the speed of the hydraulic pump. The speed control of the hydraulic pump is preferably configured such that the greater accelerator stroke, the higher pump speed. This can be applied up to a maximal pump speed that cannot be exceeded independently of the accelerator stroke. The restricted pump speed will save fuel for two main reasons; first, for a given flow the pump has usually higher efficiency for greater pump displacement and lower speed than for higher speed and smaller pump displacement, and second, if the operator is forced to use the maximal lever stroke in operating the hydraulics, the pressure drop over the control valve will be reduced due to an increased control valve opening. In addition, an “on-off” control of the hydraulics will facilitate the driving and give the operator more ergonomic conditions.
According to one embodiment of the invention, the drive source of the hydraulic pump is the internal combustion engine and the accelerator is arranged for controlling the speed of the hydraulic pump by controlling the speed of the internal combustion engine. According to another embodiment of the invention, the working machine comprises a further electric motor and the drive source of the hydraulic pump is said further electric motor, and the accelerator is arranged for controlling the speed of the hydraulic pump by controlling the spud of said further electric motor. For both these variants, the pump speed can be controlled and restricted in a way that is similar to the characteristics of a conventional working machine where both the hydraulic pump and the driveline are directly driven by an internal combustion engine.
According to a further embodiment of the invention, the working machine has a means for determining the speed of the drive line of the working machine, and the speed control of the hydraulic pump is dependent on the determined drive line speed. Hereby, the pump speed is not only dependent on the accelerator stroke, but also on the speed of the driveline. This means that the pump speed as a function of the accelerator stroke can be varied for different speeds of the driveline. In a diagram showing pump speed versus accelerator stroke, different curves with different maximal pump speed for instance, can represent different driveline speed.
For example, the gear position of the working machine can be determined, and the speed control of the hydraulic pump can be dependent on the determined gear position. The current gear position gives a non-complicated robust indication of the speed of the driveline. For example, if the gear position is neutral, the allowed maximal pump speed can be relatively high enabling a fast lift operation by means of the hydraulics while the power to the driveline is zero or negligible.
According to a further embodiment of the invention, the working machine has a means for determining the power or torque transmitted by the drive line of the working machine, and the speed control of the hydraulic pump is dependent on the determined power or torque. Hereby, the pump speed is not only dependent on the accelerator stroke, but also on the power consumed by the driveline. Since the internal combustion engine speed in a conventional machine is depressed if a lot of power is transmitted to the driveline, the pump speed is also restricted. Thus, this embodiment of the invention will enable the pump speed to be controlled similar to the restricted pump speed available in a conventional machine under the same operation conditions.
According to a further embodiment of the invention, the working machine has a means for determining activation of a braking device of the working machine, and the speed control of the pump is dependent on whether or not the braking device has been activated. The hydraulic pump is preferably configured such that the maximal pump speed is higher for an activated braking device than for an inactivated braking device. Hereby the pump speed can be controlled similar to the restricted pump speed available in a conventional machine under the same operation conditions. For example, the operator may increase the pump speed temporarily by activating the brake pedal at the same time as the accelerator, is activated. The increased pump speed could be further conditioned by a certain gear and/or velocity of the working machine.
According to a further aspect, the invention relates to a control element for a working machine, where the working machine has a hydraulic pump for driving a hydraulic actuator, and the control element comprises means for selecting between machine performance and fuel save by an operator of the working machine, and where the control element is designed such that a selected increased machine performance and a decreased fuel save correspond to a higher available speed of the hydraulic pump and a selected decreased machine performance and an increased fuel save correspond to a lower available speed of the hydraulic pump. Such a control lever can be arranged on a switch panel inside the cabin of the working machine. The operator is hereby able to prioritize machine performance or fuel save depending on the current working operation to be performed. For example, in a working machine where the speed control of the hydraulic pump is configured such that the greater accelerator stroke, the higher pump speed, and up to a maximal pump speed that cannot be exceeded independently of the accelerator stroke, the selection of machine performance or fuel save will displace the pump speed curve (that is a function of the accelerator stroke) towards increased and decreased pump speed, respectively.
The same advantages as discussed above with reference to the working machine can be reached by the method and the control unit according to the invention, and the control element according to the invention. Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.